The invention relates generally to access networks, and more particularly to a passive optical access network.
The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. Although the bandwidth of backbone networks has experienced a substantial growth in recent years, the bandwidth provided by access networks has remained relatively unchanged. Thus, the xe2x80x9clast milexe2x80x9d still remains a bottleneck between a high capacity LAN or Home network and the backbone network infrastructure.
Digital Subscriber Line (DSL) and Cable Modem (CM) technologies offer some improvements over more conventional last mile solutions. However, these technologies still do not provide enough bandwidth to support emerging services such as Video-On-Demand (VoD) or two-way video conferencing. In addition, not all customers can be covered by DSL and CM technologies due to distance limitations.
One broadband access network architecture that offers a solution to the xe2x80x9clast milexe2x80x9d problem is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splitters and combiners to passively distribute optical signals between the OLT and the remote ONUs.
FIGS. 1A and 1B illustrate the management of network traffic in a point-to-multipoint PON. As an example, the PON is shown to include an OLT 102 and three ONUs 104, 106 and 108, although the PON may include additional ONUs. Referring to FIG. 1A, the OLT includes an optical transmitter 110 that sends downstream traffic containing ONU-specific information blocks 1, 2 and 3 to the ONUs. The downstream traffic is optically broadcasted by a passive optical splitter/combiner 112 into three separate signals that each carries all of the ONU-specific information blocks. The ONUs 104, 106 and 108 include optical receivers 114, 116 and 118, respectively, that receive all the information blocks transmitted by the OLT. Each ONU then processes the information blocks that are intended for that ONU and discards the information blocks that are intended for the other ONUs. For example, ONU-1 receives information blocks 1, 2, and 3. However, ONU-1 only delivers information block 1 to end user 1. Likewise, ONU-2 only delivers information block 2 to end user 2 and ONU-3 only delivers information block 3 to end user 3.
Referring to FIG. 1B, the ONUs 104, 106 and 108 also include optical transmitters 120, 122 and 124, respectively, to transmit upstream traffic to OLT 102. The upstream traffic is managed utilizing a time division multiplex access (TDMA) protocol, in which specific transmission time slots are dedicated to individual ONUs. The ONU-specific time slots are synchronized so that upstream information blocks from the ONUs do not interfere with each other once the information blocks are combined onto the common fiber. For example, ONU-1 transmits information block 1 in a first ONU-specific time slot, ONU-2 transmits information block 2 in a second ONU-specific time slot, and ONU-3 transmits information block 3 in a third ONU-specific time slot. The time division multiplexed upstream traffic is then received by an optical receiver 126 of the OLT.
A concern with a TDMA PON is that the bandwidth of the PON is not always efficiently utilized for data transmission because network traffic typically exhibits a high degree of burstiness. The bursty network traffic results in some transmission time slots that consistently overflow even under a very light traffic load. In addition, the bursty network traffic results in some transmission time slots that are not completely filled even when the overall traffic load is very high.
In view of the above concern, there is a need for an access network based on PON technology that efficiently utilizes the available bandwidth by reducing the amount of bandwidth wasted because transmission time slots are not filled to the maximum capacity.
An optical access network and method for transmitting optical data in the network utilizes an interleaved polling scheme to efficiently use the available bandwidth of the network. The use of the interleaved polling scheme allows a central terminal of the network to dynamically allocate upstream bandwidth from remote terminals of the network to the central terminal in response to the amount of data that is waiting at the remote terminals to be transmitted to the OLT. In one embodiment, the optical access network is based on Passive Optical Network (PON) technology. In another embodiment, the optical access network utilizes Ethernet protocol to encapsulate data in Ethernet frames for transmission. Thus, in these embodiments the optical access network includes all of the advantages associated with the PON technology and/or the Ethernet protocol. In addition, since the allocation of upstream bandwidth is on an as needed basis, loss of bandwidth due to unfilled time slots is substantially eliminated.
A method of transmitting optical data in an optical network in accordance with the invention includes the steps of generating a table that includes information about the current sizes of data waiting to be transmitted from a plurality of remote terminals to a central terminal, selectively transmitting grant messages to the remote terminals, receiving authorized amounts of the data from the remote terminals and request messages containing updated information about the current sizes of the data waiting to be transmitted at the remote terminals in response to the grant messages, and updating the table using the updated information contained in the request messages received from the remote terminals. Each grant message is indicative of a permission for a targeted remote terminal to transmit an authorized amount of data waiting at the targeted remote terminal, which is dependent on the information included in the table with regard to the targeted remote terminal. In an embodiment, the optical network is a passive optical network (PON), which may be an Ethernet-based PON.
In an embodiment, the table that-includes information about the current sizes of data waiting to be transmitted from the plurality of remote terminals further includes information about round trip times of data transmission between the central terminal and the remote terminals. In this embodiment, the method may include steps of computing current round trip times for the remote terminals, including monitoring transmission times associated with the grant messages from the central terminal and reception times associated with the authorized amounts of the data from the remote terminals, and updating the information about round trip times of data transmission using the computed current round trip times.
In an embodiment, the step of updating the table includes subtracting a value from a new entry for the targeted remote terminal. The value corresponds to the actual amount of data transmitted from said targeted remote terminal, while the new entry corresponds to the updated information contained in a request message from the targeted remote terminal.
In an embodiment, the method may further include a step of scheduling transmission times for the grant messages such that the data and the updated information from the remote terminals do not overlap during transmission. The transmission times for the grant messages substantially define receptions times for the request messages at the central terminal. The step of scheduling transmission times may include rescheduling an original transmission time for a grant message to a rescheduled transmission time when the original transmission time conflicts with anther transmission time for a different grant message.
In an embodiment, the method further includes steps of detecting a disconnected remote terminal, and reducing the frequency of grant messages that are transmitted to the disconnected remote terminal. The step of detecting a disconnected remote terminal may include waiting a predefined period for the disconnected remote terminal to respond to a grant message transmitted to the disconnected remote terminal.
An optical access network in accordance with the invention includes a plurality of remote terminals and a central terminal. In one embodiment, the optical access network is an Ethernet-based PON. The remote terminals of the network receive and transmit optical data. Each remote terminal is configured to transmit a request message and an authorized amount of data waiting at the remote terminal in response to a grant message received by the remote terminal. The request message includes updated information about the current size of data waiting at the remote terminal, while the grant message includes information that indicates the authorized amount. The central terminal of the network is optically coupled to the remote terminals to transmit and receive the optical data.
The central terminal includes memory that has a table containing latest information about the current sizes of data waiting to be transmitted from the remote terminals to the central terminal, and a processor that is configured to selectively transmit grant messages to the remote terminals using the latest information contained in the table to indicate authorized amounts of data that can be sent by receiving remote terminals. The processor of the central terminal is further configured to receive request messages from the remote terminals in response to the grant messages and to update the table in memory using the updated information contained in the request messages.
In an embodiment, the processor of the central terminal is configured to update the table in memory by subtracting values from new entries for selected remote terminals that have transmitted the request messages to the central terminal. The values correspond to the actual amounts of data transmitted from the selected remote terminals, while the new entries correspond to the updated information contained in the request messages.
In an embodiment, the processor of the central terminal is further configured to schedule transmission times for the grant messages such that the data and the request messages from the remote terminals do not overlap during transmission. In this embodiment, the processor may be further configured to reschedule an original transmission time for a specific grant message to a rescheduled transmission time when the original transmission time conflicts with another transmission time for a different grant message.
In an embodiment, the processor of the central terminal is configured to detect disconnected remote terminals by identifying the remote terminals that have not responded to the grant messages within a timeout period. In this embodiment, the processor may be configured to reduce the frequency of grant messages that are transmitted to the disconnected remote terminals.
In an embodiment, the table in memory further includes information about round trip times of data transmission between the central terminal and the remote terminals. In this embodiment, the processor of the central terminal may be configured to compute current round trip times for the remote terminals by monitoring transmission times associated with grant messages from the central terminal and reception times associated with request messages from the remote terminals and to update the information about round trip times using the current round trip times.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.